This invention relates to a reference signal reproduction apparatus for digitally reproducing a reference signal that is synchronous with an arriving reference signal.
Teletext systems in which digital signals are superposed in the vertical blanking interval of the television signal, and during the horizontal scan period, in which normally no picture signals are sent, have been developed. On the receiving side of such a teletext system, image data such as characters and graphics included in a superposed digital signal (hereinafter referred to as a character transmission signal) are extracted and displayed on the CRT, etc. In general, the horizontal synchronizing signal of the television signal is used as the reference for extracting the above character broadcasting signal.
In order to obtain this horizontal sychronizing signal, a synchronizing AFC circuit, which is described in pages 168 to 176 of the COLOR TELEVISION TEXTBOOK, Oct. 20, 1978, Japan Broadcasting Publishing Assoc. was used in the past. The horizontal synchronizing signal reproduced by this circuit however, does not have accurate phase in the vertical blanking interval because of the effect of the vertical synchronizing signal. Consequently, when the superposed character broadcasting signal included in the vertical blanking interval is extracted, it is not possible to correctly extract the character broadcasting signal using the horizontal synchronizing signal reproduced by the synchronizing AFC circuit.